1. Field of the Invention
The present invention relates to a high frequency amplifier, more particularly, to a high frequency amplifier having a high frequency transistor used for a wireless installation, a portable telephone, and the like.
2. Description of the Related Art
Recently, a high frequency transistor such as a silicon (Si) transistor or a gallium arsenide (GaAs) field effect transistor (FET) is used for an amplifier element of a communication device in response to the requirements for high performance and miniaturization. A high frequency amplifier device using the above amplifier elements is constituted by connecting a plurality of single high frequency amplifiers for a multistage amplification, so that a required amount of amplification is obtained.
The (single) high frequency amplifier is, for example, constituted by an amplifier element (transistor), an input matching circuit, an output matching circuit, a bias supply circuit, and the like. These circuits are sufficiently matched (generally, 50 .OMEGA.) in a specific high frequency band used for the high frequency amplifier device. Nevertheless, in the other frequency bands except for the specific high frequency band, the matching characteristics of the circuits are not sufficient, that is, the matching characteristics thereof are not specified at 50 .OMEGA.. Therefore, when connecting a plurality of high frequency amplifiers each having a single transistor to constitute a high frequency amplifier device by a multistage amplification, an input impedance or an output impedance of the high frequency amplifier device including a plurality of high frequency amplifiers may be positioned in an unstable frequency area due to the physical property of the transistor, so that stable amplification by each of the high frequency amplifiers may be adversely affected and the high frequency amplifier device begin to oscillate.